A controlled resistance welding process should maintain the correct current density during the weld current flow time, the correct forge pressure established by the constriction of the work pieces between the opposed electrodes and proper quench cycle as the electrodes are maintained in contact with the work following the weld current cycle. Each of these factors are related to the secondary weld current.
The quality of work or parts produced by a resistance welding machine is dependent in part on maintaining a consistent level of various supply systems or making corrections within a range of acceptable values and on the interaction of various electromechanical devices during the machine's sequence cycle.
The part area in contact that can be joined during the welding operation is a function of the applied electrode force and the geometric form, size and hardness of the part weld projection relative to the weld current flowing to enable achievement of the correct current density. The standard equation is I.sup.2 Rt; however, to be a more accurate expression of the current density the relationship is expressed as I.sup.2 Rt/A.sup.2, where I is the curret, R is the resistivity, t is the time and A is the weld area such as projection area in contact with the other work piece. It is a common practice in the art to seek control of the secondary weld current (I.sup.2) and the duration of the weld current flow (t). To effectively control the overall welding cycle and weld quality it is also necessary to correct the contact area for the amount of weld current available or flowing or to adjust some welding cycle parameter to accommodate variations in the contact area.